In ion cyclotron resonance mass spectrometers (ICR-MS), the mass-to-charge ratios m/z of ions are measured by their cyclotron movements in a homogeneous magnetic field with high field strength. The magnetic field is usually generated by superconductive magnetic coils cooled in liquid helium. Nowadays, they provide usable cell diameters of around 6 to 12 centimeters at magnetic field strengths of 7 to 12 Tesla.
The orbital frequency of the ions (ion cyclotron frequency) is measured in ICR measuring cells located within the homogeneous part of the magnetic field. The ICR measuring cells normally comprise four longitudinal electrodes which extend in a cylindrical arrangement parallel to the magnetic field lines and surround the measuring cell like a sliced sleeve. Usually, two of these electrodes are used to bring ions, introduced close to the axis, into their cyclotron orbits (into their cyclotron motion), ions with the same mass-to-charge ratio being excited as in phase as possible in order to obtain a synchronously orbiting cloud of ions. The two other electrodes serve to measure the orbiting of the ion clouds by their image currents, which are induced in the electrodes as the ions fly past. The term “image currents” is normally used even though it is actually the induced “image voltages” which are measured. The processes of introducing the ions into the measuring cell, ion excitation and ion detection are carried out in successive steps of the method.
Since the mass-to-charge ratio of the ions (referred to below simply as “specific mass”, and sometimes simply as “mass”) before the measurement is unknown, the ions are excited by a complete and homogeneous mixture of excitation frequencies. This mixture can be a temporal mixture with frequencies increasing with time (called a “chirp”), or it can be a synchronous, computer-calculated mixture of all frequencies (a “sync pulse”). By specially selecting the phases, the synchronous mixture of the frequencies can be formed so that the amplitudes of the mixture remain restricted to the dynamic region of the digital-to-analog converter, which produces the temporal analog voltage sequence characteristics for the mixture.
The image currents induced by the ions in the detection electrodes are amplified, digitized and analyzed by Fourier analysis for the orbital frequencies present therein. The Fourier analysis transforms the original measurements in the “time domain” into a “frequency domain”, hence the term Fourier transform mass spectrometry (FTMS). The specific masses of the ions and their intensities are then determined from the signals, which can be recognized as peaks in the frequency domain. Owing to the extraordinarily high constancy of the magnetic fields used, and the high accuracy for frequency measurements, it is possible to achieve an extraordinarily accurate mass determination. At present, Fourier transform mass spectrometry is the most accurate of all types of mass spectrometry. Ultimately, the accuracy depends only on the number of ion orbits which can be detected by the measurement.
The longitudinal electrodes usually form a measuring cell with a square or circular cross-section. The cylindrical measuring cell contains four cylinder segments as longitudinal electrodes. Cylindrical measuring cells are the ones most commonly used because they produce the best utilization of the magnetic field, although the image currents of focused clouds of ions with the same mass (image voltages) come close to a rectangular curve.
Since the ions can move freely in the direction of the magnetic field lines and possess, from the filling phase, all velocity components in the direction of the magnetic field, they must be prevented from leaving the measuring cell. To prevent ion losses, the measuring cells are therefore equipped at both ends with electrodes, known as “trapping electrodes”. These are supplied with ion-repelling DC voltage potentials in order to keep the ions in the measuring cell. There are widely differing forms for this electrode pair, the simplest being planar electrodes with a central aperture. The aperture serves to introduce the ions into the measuring cell.
The ion-repelling potentials form a potential well in the interior of the measuring cell, with a parabolic potential profile along the axis of the measuring cell. The potential profile is only weakly dependent on the shape of these electrodes. The potential profile along the axis is at its minimum at precisely the mid-point of the measuring cell if the ion-repelling potentials across both electrodes have the same value. The ions introduced will therefore execute oscillations in this potential well in the axial direction—so-called trapping oscillations—because they still posses kinetic energy in the axial direction from their introduction. The amplitude of the trapping oscillations depends on their kinetic energy.
The electric field outside the axis of the measuring cell is more complicated to describe. It inevitably contains field components in the radial direction which generate a second type of ion motion: magnetron circular motion. The magnetron gyration is also a circular motion about the axis of the measuring cell, but much slower than the cyclotron circular motion. The additional magnetron circular motion causes the mid-points of the cyclotron circular movements to gyrate around the axis of the measuring cell at the frequency of the magnetron motion, with the result that the trajectory of the ions describes a cycloidal motion.
The superposition of magnetron and cyclotron circular motion is an undesirable phenomenon which leads to a frequency shift in the cyclotron frequency. Furthermore, it leads to a reduction in the usable volume of the measuring cell. The measured frequency ωm (the “reduced cyclotron frequency”) amounts to
            ω      m        =                            ω          c                2            +                                                  ω              c              2                        4                    -                                    ω              t              2                        2                                ,where ωc is the undisturbed cyclotron frequency, and ωt the frequency of the trapping oscillation. The trapping oscillation determines the effect of the magnetron circular motion on the cyclotron circular motion. A measuring cell without magnetron circular motion would be very advantageous because the cyclotron frequency could be directly measured and no corrections would have to be applied.
The vacuum in the measuring cell must be as good as possible because, during the measurement of the image currents, the ions should not collide with molecules of residual gas. Each collision of an ion with a molecule of residual gas brings the ion a bit out of the orbiting phase of the other ions with the same specific mass. The loss of phase homogeneity leads to a reduction in the image currents and to a continuous decrease in the signal-to-noise-ratio, which reduces the usable measuring period. The measurement period should amount to at least a few hundred milliseconds, ideally a few seconds. This requires a vacuum in the region of 10−7 to 10−9 Pascal.
Apart from the vacuum, the space charge in the ion cloud can also adversely affect the measurement. The Coulombic repulsion between the ions themselves and, above all, the elastic reflection of the ions moving in the cloud lead to a plurality of disturbances, which also lead to an expansion of the cloud. In present-day instruments, the space charge, alongside the effects of pressure, represents the greatest limitation on achieving a high mass accuracy.
For higher specific ion masses, the decrease in the cyclotron orbital frequency of the ions is inversely proportional to the mass. The resolution, however, is proportional to the number of measured orbits; it is therefore lower for ions of high specific masses than for those of low specific masses, although a high resolution and, correspondingly, a high mass accuracy is particularly desirably for ions of high masses. Since the introduction of ion cyclotron mass spectrometers, repeated attempts have been made to increase the resolution, particularly for higher specific ion masses, by using a larger number of detection electrodes to increase the frequency of the image currents in relation to the cyclotron frequency. If a total of sixteen detection electrodes are used instead of two, then the image currents are each measured sixteen times instead of two times, and the measured frequency increases by a factor of eight. It is to be expected that resolution and mass accuracy are also increased by a factor eight if measured over the same measuring time.
Unfortunately, these experiments have had only moderate success, and so they have regularly been abandoned. The reasons for the moderate success have not been adequately explained. It can be assumed that the ion clouds do not hold together well enough and that, for this reason, they cannot be brought close enough to the detection electrodes. Narrow electrodes require that the ion clouds are brought up very close to the detection electrodes, since otherwise it is scarcely possible to induce the image currents at full strength.
Recently, measuring cells for ion cyclotron resonance mass spectrometry have been elucidated in which practically no magnetron circular motion can develop. (E. Nikolaev, Lecture at the International Mass Spectrometry Conference (IMSC) in Edinburgh, September 2003). In this case, the trapping electrodes are replaced with fine electrode structures, to which an RF voltage is applied and which thus reflect ions of both polarities because of their pseudopotential if the ions possess a specific mass above a mass threshold. The mass threshold can be adjusted by the RF voltage. Electrode structures of this type have been elucidated in U.S. Pat. No. 5,572,035 (J. Franzen). The pseudopotential has a very short range of the order of magnitude of the widths of the structural elements of this electrode structure. The reflection resembles a hard reflection on a matt screen, the scattering effect of the matt screen decreasing as the angle of incidence flattens out.
An RF field around the tip of a wire decreases outwards proportional to 1/r2; the RF field of a long wire decreases at 1/r, where r is the distance from the tip or axis of the wire. Both RF fields repel both positive and negative particles. The particle oscillates in the RF field. Regardless of its charge, it experiences the strongest repelling force when it is located near to the wire, i.e. at the point where the field strength is highest. It experiences the strongest attractive force when it is at the furthermost point, i.e. at the point on its oscillation path where the field strength is lowest. Integration over time results in a repulsion. This time-integrated repulsion potential is known as the “pseudopotential”, sometimes also as the “effective potential” or “quasi-potential”. The pseudopotential is proportional to the square of the RF field, i.e. it decreases outwards at 1/r2 in the case of a long wire. Moreover, the pseudopotential is inversely proportional to the specific mass m/z of the particles and to the square ω2 of the RF frequency ω. There is a lower mass threshold for the reflection of the particles.
The relatively easily manufactured surface, made of a grid of parallel wires, already has a very short range pseudopotential. The RF field of a grid with wires of 0.1 millimeter, one millimeter apart, falls to 5% in one millimeter, to 0.2% in two millimeters and to 0.009% in three millimeters. The pseudopotential, which is proportional to the square of this field, falls off much more quickly: At a distance of one millimeter, there is a pseudopotential of only 0.25%.
The ions are stored in these new measuring cells in the form of a fine ion string with no magnetron motion. Owing to their kinetic energy, the ions can move to and fro in the axial direction in the ion string; they undergo hard reflection at each of the trapping electrodes, and the slightly scattering reflection leads to minuscule cyclotron helical movements of the ions. The ion strings as a whole can now be excited via suitable chirp or sync pulses so that they perform a cyclotron circular motion. In the orbiting ion string, the scattering effect of the reflections also decreases, so that the diameter of the ion string only increases very slowly. These long ion strings can consist of significantly more ions than previous measuring cells without the space charge adversely affecting the cyclotron circular motion. The space charge also allows the diameter of the ion string to increase only very slowly.